Engine knock (also called detonation or pinging) occurs in a flame propagation engine when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. This auto ignition occurs rapidly, possibly creating a detonation wave that leads to an audible ping or knock. This knock phenomena is affected by the engine's intake manifold pressure, the engine compression ratio, and the spark advance. All of these factors affect engine performance, emissions, and efficiency.
Engine lubricating oil can contribute to knock when used either as a fuel or conventionally for lubrication. For example, U.S. Pat. No. 7,389,752, entitled “Use of Engine Lubricant as Ignition Fuel for Micro-Pilot Ignition System of an Internal Combustion Engine, to Terrence Alger, et al., describes the use of lubricating oil as fuel in a pilot ignition system. U.S. Pat. No. 7,262,155, entitled “High Octane Lubricants for Knock Mitigation in Flame Propagation Engines”, to Thomas Ryan, et al, describes how engine lubricant oil may be formulated to reduce knock when used conventionally to coat the engine cylinders.
There is much research directed toward reducing knock. Compositions of fuels and lubricants and methods to prevent or reduce such knock in flame propagation engines are highly desirable. Engine-based testing is one approach to determining the effect of lubricant oils on knock. However, this type of testing is expensive and can introduce complicating factors such as oil consumption and heat transfer.